EMI filters are routinely provided with electrical and electronic devices to ensure that any electromagnetic noise generated by the device is not allowed to pass back into the main power supply. Generally such filters are classified as being either differential mode (DM) filters comprising inductive elements in the live and neutral lines, or common mode (CM) filters comprising capacitive elements between the live and neutral lines and ground.
Traditionally EMI filters are constructed by conventional circuit components including in particular discrete capacitive and inductive components provided on a circuit board. However, with increasing miniaturization of electronic components such EMI filters may become disproportionately large with respect to the associated circuits and components that are being filtered. There is therefore a need for more compact EMI filter designs, and in particular for EMI filters that can be integrated with other electronic components in an electronic device.
Known in the art are coreless planar spiral windings (CPSW) that may be used in coreless transformers. A CPSW may be formed in a number of ways, for example by depositing conductive tracks on a printed circuit board and when used in a coreless transformer for signal and/or power transfer primary and secondary windings may be formed as CPSWs on opposing sides of a suitable dielectric substrate. In addition to their use in coreless transformers for power and signal transfer, such planar spiral windings have also been proposed for use in integrated EMI filters with the help of ferrite materials. Coupled coreless planar spiral windings have also been proposed to cancel the capacitor parasitic inductance in traditional EMI filters. It has also been known to use CPSW as the inductive elements in differential mode EMI filters, but such prior proposals have always continued to use discrete three-dimensional capacitors that prevent complete integration of the EMI filter structure.